The present invention is directed to a coherent optical multi-channel arrangement composed of a plurality of spatially distributed optical transmitters which will transmit optical information signals that are frequency-modulated with a modulation frequency from a predetermined modulation frequency range, said transmitters transmitting said optical information signals on carrier frequency channels with channel carrier frequencies that differ from one another with such a small channel frequency spacing that crosstalk can occur due to transmitter instabilities, said plurality of transmitters forming one or more groups spatially separated from one another composed of, respectively, a plurality of spatially-adjusted transmitters, wherein a coherent optical heterodyning arrangement is provided for receiving a part of the information signal emitted by the transmitters on the carrier frequency channel and generate an optical heterodyne signal by superimposing this part with the light of a reference frequency that is fixed in terms of frequency and lies at the channel carrier frequency of the information signal, the reference frequency line being from an optical frequency comb spectrum composed of a plurality of reference frequency lines that are fixed in terms of frequency and have fixed reference frequencies that lie at different channel carrier frequencies and are arranged at a slight channel frequency spacing from one another, the arrangement includes a supervisory arrangement provided for controlling the channel carrier frequencies of every optical transmitter, dependent on the heterodyne signal generated by superimposing the part of the information signal emitted by said transmitter with the channel carrier frequency of the transmitter with the light of the reference frequency line of the reference comb spectrum so that a frequency change of the channel carrier frequency in this heterodyne signal will be cancelled.
A coherent optical multi-channel arrangement is disclosed in an article by Ishida et al entitled "0.04 Hz Relative Optical-Frequency Stability in a 1.5 .mu.m Distributed-Bragg-Reflector (DBR) Laser", IEEE Photonic Technology Letters, Vol. 1, No. 12, December 1989, pp. 452-454. In order to avoid a crosstalk between the carrier frequency channels having the small frequency spacing, for example 5 GHz through 10 GHz, in this arrangement, a separate coherently optical heterodyning or superimposing means is provided for each of the transmitters for the stabilization of the frequency spacing of the individual transmitters so that the same number of such heterodyning means as transmitters are provided.
In this known arrangement, the reference laser generates a reference comb spectrum, for example with the assistance of an external modulator, so that the reference frequency lines of this reference comb spectrum are arranged at a frequency spacing from one another that is the same as the slight frequency spacing between the carrier frequency channels. A part of the optical information signal emitted by the transmitter is superimposed with the light of the reference frequency line of the reference comb spectrum belonging to the particular transmitter in every heterodyning means, for example a heterodyne receiver, which is allocated to the transmitter, so that the fixed reference frequency of this reference frequency line is the same as the channel carrier frequency of the carrier frequency channel of the transmitter. Via a phase or frequency control loop, the channel carrier frequency of the transmitting laser is readjusted to give a frequency deviation wherein the intermediate frequency contained in the heterodyne signal due to the superimposition of the part of the information signal of the transmitter with the light of the appertaining reference frequency line of the heterodyning means belonging to this transmitter remains constant, and this intermediate frequency is then equal to a difference between the channel carrier frequency of the transmitter and the reference frequency of the reference frequency line.
Since a separate heterodyning means is provided for every transmitter in this known arrangement, a considerable outlay for frequency control is necessary given such an arrangement with many transmitters. Many transmitters will be provided given a space and frequency switching or a TV distribution system having coherent technology.